1. Field of the Invention
The present invention relates to optical disc recording and playback apparatuses provided with a laser light source irradiating an optical disc with laser light and an optical pickup converting reflected light from the optical disc into a plurality of electrical signals via a split photo detector, the optical disc recording and playback apparatuses recording information on the optical disc, and reading information stored in the optical disc and playing back the information thus read. In particular, the present invention relates to an optical disc recording and playback apparatus performing recording on and playback from a DVD (Digital Versatile Disc) having two recordable layers: a zeroth layer and a first layer.
2. Description of Related Art
Conventionally, in optical disc recording and playback apparatuses provided with a laser light source irradiating an optical disc with laser light and an optical pickup converting reflected light from the optical disc into a plurality of electrical signals via a split photo detector, the optical disc recording and playback apparatuses recording information on the optical disc, and reading information stored in the optical disc and playing back the information thus read, a tracking servo method is adopted as a control method for making the laser light emitted from the optical pickup accurately follow a track formed on the optical disc. As the tracking servo method, a DPP (differential push pull) method is known, by which a tracking error signal is produced by performing computation on output signals of light receiving elements, the output signals obtained from one main beam and two sub beams reflected from the optical disc.
In order to improve accuracy of tracking servo using the DPP method, various apparatuses, methods, or the like, have been proposed. For example, JP-A-2005-228366 discloses an optical disc recording and playback apparatus in which a gain value allowable range setting portion sets the allowable range of gain values by actual measurement of gain value corresponding to ratio of level of an MPP (main push pull) signal and level of an SPP (sub push pull) signal, both of which are produced by a tracking error signal computing circuit, and an eccentricity measuring portion measures degree of eccentricity of the optical disc, and sets a gain value of an amplifier based on the degree of eccentricity thus measured.
However, although the above optical disc recording and playback apparatus can improve the accuracy of tracking servo, in case where recording is performed by using the gain value set at the time of playback, the accuracy of tracking servo may be reduced because intensity of laser light shone onto the optical disc is different when playback is performed and when recording is performed.
In particular, in a case where recording is performed on a DVD having two recordable layers: a zeroth layer and a first layer, the balance of a tracking error signal may be disrupted (see FIG. 7B) due to, for example, influence of interfering light from adjacent layers, causing instability of tracking servo.